Sympathomedullary Pathway

Sympathomedullary Pathway: Definition

The Sympathomedullary Pathway (SAM) is the body’s system for responding to acute stress (short-term stress).

The SAM responds rapidly to stressors, meaning a person can react quickly if they come under a threat. SAM was essential for our ancestors, who had to respond quickly in life-threatening situations such as coming across a lion.

Fig. 1 - Long term exposure to stress can cause illnesses.

Sympathomedullary Pathway Stages

In a stressful situation, the brain activates the fight or flight response via the SAM. Let’s have a look at the stages of this stress response system:

1. The hypothalamus (part of the brain in charge of the stress response) activates if a situation is stressful.

2. The autonomic nervous system (regulates physiological processes such as heart rate, blood pressure, respiration, and digestion), which includes the sympathetic nervous system, is aroused, and heart rate and blood pressure increase.

3. The sympathetic nervous system signals the adrenal medulla (located inside the adrenal cortex, in the centre of an adrenal gland) to release adrenaline and noradrenaline.

4. Adrenaline and noradrenaline activate the fight or flight response.

5. After the threat, the parasympathetic nervous system returns the body to a relaxed state. So, the sympathetic pathway is no longer overworking.

SAM Pathway Diagram

The SAM pathway diagram illustrates the Sympathomedullary Pathway stages.

The SAM Pathway is an adaptive stress management response. In moderate amounts, stress is useful - even essential! Stress helps us to adapt to our surroundings and new things. Stress can be lifesaving. Without the SAM Pathway, our bodies wouldn’t react to threats with the energy we need to either face a threat or flee to safety.

Built within the SAM process is a return to stasis or a resting level of calm. However, an overactive SAM can disastrously affect our bodies and mental well-being. Stress can provoke myriad physical symptoms and illnesses.

When our bodies are healthy, they can handle the extra energy required to breathe sufficiently during heightened states of arousal. However, increased respiration can weaken our system when it is prolonged.

Fig. 2 - Overactivation of the SAM pathway can adversely affect our health, especially concerning the heart.

Too much stress can even increase our chances of heart disease and hypertension. A heightened state of arousal causes our heart rate to increase and makes our heart beat harder because it causes our blood vessels to dilate to allow increased blood flow.

Too much cortisol in the bloodstream carries its own host of consequences. It can produce psychological effects, like anxiety or depression.

Cortisol, the stress hormone, causes our bodies to release stored glucose resulting in physical effects like weight gain, increased blood pressure, and even Type 2 Diabetes.

Cortisol also increases the release of gastric acid to nourish our cells and help our bodies absorb the nutrients needed for fast action. Too much of this can cause intestinal problems like indigestion, abdominal pain, diarrhoea, or vomiting.

Evidence for Sympathomedullary Pathway

Taylor et al. (2000) found that acute stress produces the fight or flight response in men but the tend-to-befriend response in women.

Also, female stress responses that increased the survival of offspring were more likely to have been passed on through natural selection. It seems there are gender differences in how people respond to acute stressors.

Age also affects a person’s physiological response to stress. McCarty (1981) subjected rats of various ages to one minute of electric shocks to the feet (acute stressor); before the electric shocks, their blood plasma levels of adrenaline and noradrenaline were the same. After the electric shocks, the older rats had lower levels of adrenaline and noradrenaline than the younger rats.

Horwatt et al. (1988) found that if animals are exposed to the same stressful stimulus every day for weeks, several adaptive changes occur in the SAM. If these animals are then exposed to a new stressful situation, they display an exaggerated SAM response compared to animals without this conditioning. The finding suggests that acute stress responses develop differently according to previous stress experiences.

Evaluating Sympathomedullary Pathway

It seems there are gender differences in how people respond to acute stressors. Thus, the acute stress response is different for each person; we should not assume everyone will react in the same way physiologically. The fight-or-flight response may be a male-centric (focused or centred on men) viewpoint.

Lots of research into the acute stress response has focused on animals. Humans are physiologically different; therefore, animal responses to acute stress and human responses are incomparable, so it raises the question of whether researchers should extrapolate results from animals to humans.

Lazarus (1999) argued that a physiological account alone could not explain how people respond to stressors. How we perceive the potential stressor (the demands of the stressor and our ability to cope with it) also determines our response.

Speisman et al. (1964) provided evidence for this argument. The researchers asked students to watch a gruesome medical procedure while measuring their heart rates. When the video became traumatic for the students to watch, their heart rates would increase, indicating the stress response. However, their heart rates decreased if they interpreted the procedure as a voluntary and joyful rite of passage, which researchers suggested at the start of the study.